MS Word can split a cell into two cells, but not Excel.
You might see if this would look presentable, the vertical line
will not touch the borders abc | def
you can reduce the row height to make it touch the borders,
which may or may not be noticeable.
The other solution would be to insert another column
then merge the cells in the two columns row by row
except for that two cells that are to be distinguished.
You can use the Merge Across toolbar button.
http://www.mvps.org/dmcritchie/excel/merge.htm#builtin